I have an open vented gravity system with no zone valves and a pump for Central Heating side only.
There are 2 tanks in loft, one is a cold water tank and the other a feed and expansion tank. Each of them have a vent pipe.
The boiler fires up when I switch on CH setting, but hot water cylinder tank is cold. On HW only setting, the boiler does not fire up at all.
I suspect the Hot Water Cylinder Coil has an airlock.
I have attached a mains connected hose to open vent pipe on the Feed and Expansion tank and waited 30 mins. There were air bubbles at the bottom of the tank and I thought this would get rid of the airlock but doesn't seemed to have worked.
Cold water has gas in it, when it warms up the gas is released, so refilling a system with new water there is often gas in the system after, I say gas as could be gas other than air.
The problem is getting the water to start circulating before the boiler turns off, in the winter when radiators are in use the thermo syphon can start slowly, Likely the boiler will need to fire up many times to get it started,
My house originally had that system, and the problem I had was if the central heating pump started up, then upstairs radiators would stay hot even when the pump switched off. Also only way to get central heating boiler to turn off, was with the programmer, I fitted a second boiler for hot water, my dad same system fitted a motorised valve.
Some systems simply did not heat water unless central heating was running.
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